Low-lying coastal plains comprised of unconsolidated infill are internally complex hydrogeological settings, due to the high level of heterogeneity in the infill material. In order to resolve the hydrogeological processes active in these complex settings, an integrated multi-disciplinary, geoscientific approach is required. This research determines quantitatively, the effects of sedimentary aquifer heterogeneity on groundwater flowpaths and groundwater processes within a heavily laterised, coastal plain setting. The study site is the Bells Creek catchment in southeast Queensland, Australia. The methodology developed in this study provides a new approach to enable the determination of groundwater flowpaths and groundwater processes at macroscale resolution within other shallow alluvial and coastal plain aquifers. The multi-disciplinary approach utilises sedimentological, geophysical, chronological and hydrogeological techniques (including hydrochemistry and groundwater flow modelling) to develop a high-resolution aquifer framework, and to determine accurately, both groundwater flowpaths and relative flow rates. Sedimentary framework is confirmed to be the principal factor controlling the distribution of aquifer permeability pathways in any given setting, and is therefore, the dominant control over groundwater flow and processes. For the Bells Creek catchment, interpretation of stratigraphic and sedimentary data allowed the compilation of a detailed sedimentary framework. This interpretation demonstrated that weathering of the low-lying arkose sandstone bedrock has developed thick lateritic profiles. Within the weathering profiles, cemented, iron-rich horizons have resisted erosion and developed raised and elongated ridges in the modern landscape, while other clay-rich weathered layers have submitted to erosion and downgraded around those iron-rich ridges. Consequently, alluvial deposition throughout the Late Quaternary has been restricted to narrow, and relatively deep valleys containing sandrich channels, and thin floodplains at shallow depth. From a hydrogeological perspective, there is significant macroscopic aquifer heterogeneity between fine-grained lateritic mixed clay layers, floodplain clays, ironcemented ferricrete horizons, and permeable sand-rich alluvial aquifers. This variability of aquifer material has created a complex subsurface arrangement of permeability pathways. Application of Ground Penetrating Radar (GPR) in this setting enabled accurate definition of alluvial channel boundaries and the high degree of connectedness within the channels themselves. Interpretation of a comprehensive GPR dataset (that covered the entire catchment) allowed refinement of the sedimentary framework previously established to develop a detailed threedimensional aquifer framework. Finite-difference groundwater modelling and particle tracking analysis (using MODFLOW and MODPATH) has clearly demonstrated that the macroscopic heterogeneity within the various aquifer materials of the plain has marked impacts on groundwater pathways, and especially groundwater travel times. The variability between a maximum residence time of 18 months for groundwater within the alluvium, compared to hundreds of years for groundwater within the mixed clay layers of the laterite, clearly demonstrates the importance of accurately defining the spatial distribution of the various aquifer materials in a groundwater flow investigation. In this setting, the interconnection of the narrow alluvial channels feeding into a deeper alluvial delta has provided an effective conduit for shallow groundwater flow. The role of the alluvial delta in discharging the bulk of fresh groundwater from the central plain into the coastal and estuarine aquifers to the east, is certainly critical in preventing saline intrusion from encroaching further west. Hydrochemical and isotopic indicators have identified the dominant recharge processes and groundwater flowpaths within the plain, and indicated that the processes are strongly related to sub-surface permeability distributions determined in the aquifer framework (and groundwater modelling), as well as seasonal fluctuations in rainfall. In the northwest of the plain, sandstone hills provide a delayed and slightly mineralized component of groundwater recharge into adjacent highly permeable, unconfined alluvial aquifers; these aquifers also recharge directly via precipitation. Aluminosilicate weathering in the bedrock hills and eastern peripheries of the laterised bedrock are a source of excess Na, SiO2, and HCO3 to the alluvial groundwater. As this groundwater flows down-gradient to the east, however, its chemical composition evolves by sulfate reduction, silica equilibrium and ion exchange processes into a more mature Na-Cl type. Within the shallow coastal aquifers proximal to the eastern shoreline, sulfate enrichment is occurring (associated with increases in Ca, HCO3, Fe and Al) resulting in major deterioration in groundwater quality. The deterioration is produced by saline intrusion from the adjacent estuary coupled with oxidation of sulfide materials in shallow marine and estuarine clays. Reverses in salinity in those coastal aquifers have been correlated with surges in fresh recharge waters from unconfined coastal dunes and semi-confined landward alluvium, following significant rainfall events. The multi-disciplinary methodology developed, provides an effective approach for accurately defining the three-dimensional distribution of shallow aquifer material of varying permeability via detailed stratigraphic interpretation and GPR analysis. Utilising this aquifer framework, finite-difference groundwater modelling aided by hydrogeological data and hydrochemical analysis, allows accurate determination of groundwater flowpaths and groundwater processes. This research provides a new hydrogeological analogue for alluvial channel aquifers within a laterised coastal plain setting. Key Words: groundwater flow, aquifer heterogeneity, numerical modelling, hydrochemistry, recharge, ground penetrating radar, coastal plain aquifers, weathering, alluvial channels.
| Identifer | oai:union.ndltd.org:ADTP/265159 |
| Date | January 2005 |
| Creators | Ezzy, Timothy Robert |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Timothy Robert Ezzy |
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